Fluoroelastomers, and more particularly, perfluoroelastomers are materials known for their high levels of chemical resistance, plasma resistance, acceptable compression set resistance and satisfactory mechanical properties. Fluoroelastomers have thus found use as seals, gaskets and linings. When high temperature or aggressive or harsh environments, such as corrosive fluids, solvents, lubricants, and oxidizing or reducing conditions are implicated, perfluoroelastomers are the materials of choice. Fluoroelastomers are made by various routes using fluorinated monomers. Perfluoroelastomers are typically formed by using perfluorinated monomers, including a perfluorinated curesite monomer, polymerizing the monomers and curing (cross-linking) the composition using a curing agent which reacts with the incorporated curesite monomer to form a material which exhibits elastomeric properties. Suitable curesite monomers include, among others, those having cyano curesites. Examples of primary and secondary cyano-containing curesite monomers are known in the art. It is believed that in curesite monomers having cyano curesites, certain curing agents trimerize the cyano cure sites which join to form triazines.
Known curing agents include organometallic compounds and the hydroxides thereof, especially organotin compounds, including allyl-, propargyl-, triphenyl- and allenyl tin and the hydroxides. The tetraalkyltin compounds or tetraaryltin compounds, for example tetraphenyltin, are common. However, these curing agents provide a relatively slow rate of cure, are toxic and can introduce metallic contaminants to resulting elastomers.
Curing agents containing amino groups have also been employed. Bisaminophenols, bisaminothiophenols and bisamidrazones are additional types of curing agents. Those having a diphenyl structure having substitutions on each phenyl ring of amino and hydroxyl, diamine, and amino and thio are generally known in the art as being connected by structures including: —SO2—, —O—, —CO—, alkyl groups of 1-6 carbon atoms, and a carbon-carbon double bond. While perfluoroalkyl groups of 1-10 carbon atoms have been loosely described, actual synthesis and use of such compounds as curatives have not been demonstrated. Those diphenyl structure type materials which are in use and have known syntheses, are primarily compounds which have three carbon alkyl groups and in which the phenyl groups are attached to the central (second) carbon in the bis-position. For example, the most well known curative of this type is 2,2-bis[3-amino-4-hydroxyphenyl]hexafluoropropane, also known as diaminobisphenol AF or BOAP.
BOAP is a crystalline solid with a melting point of about 245-248° C. BOAP is not very compatible with perfluoroelastomers, is difficult to disperse rapidly and uniformly with perfluoroelastomers, and is thus a relatively slow-acting curative.
R. C. Evers, J. Polym. Sci. 16, 2833-2848 (1978) describe use of fluorocarbon ether bisaminophenols as monomers for making fluorocarbon ether-bibenzoxazole polymers. Evers outlined a synthesis route for the fluorocarbon ether bisaminophenols with α, ω-diiodofluorocarbon ethers as intermediates. U.S. Pat. No. 2,676,985 of Husted, Reilly & Brown in JACS 78:6032 (1956), Grigas and Taurins, Can. J. Chem., vol. 39, 414-419 (1961) and Grigas and A. Taurins, Can. J. Chem., vol. 39, 761-764 (1961) describe previously known synthesis routes for formation of amidines.
With respect to ways to speed up slow curing agents, such as BOAP, there are also traditional accelerators used in the art including organic or inorganic ammonium salts, e.g. perfluorooctanoate, ammonium perfluoroacetate, ammonium thiocyanate, and ammonium sulfamate; urea; t-butyl carbamate; acetaldehyde ammonia; tetraalkylphosphonium salts, tetraalkylammonium salts, and trialkylsulfonium salts, such as benzyltriphenylphosphonium chloride, benzyltriphenylphosphonium bromide, benzyltriphenylphosphonium phenolate of bisphenol AF, tetrabutylammonium hydrogen sulfate, and tetrabutylammonium bromide. However, such compounds tend to have side reactions that can result in undesirable byproducts.
Accordingly, there remains a need in the art for an improved curing agent capable of more easily dispersing in and more quickly curing perfluoroelastomers, particularly cyano curable perfluoroelastomers. There is further a need in the art for a cure accelerator for perfluoroelastomer curatives which accelerate the cure rate of and maintain the beneficial properties of perfluoroelastomers.